As oil companies search further into remote regions and to greater depths in their attempts to maintain adequate oil supplies, they encounter ever-increasing amounts of unusable natural gas, which consists mostly of methane. The upgrading of this natural gas to value added products, such as easily transportable fuels, is driven by the abundance of natural gas discoveries in remote areas and the disparity in prices of petroleum liquids versus gas on a BTU cost basis. Over the past several years, extensive efforts have been focused on both the direct and indirect conversion of methane to value-added products, particularly easily transportable fuels. The direct conversion routes involve some form of partial oxidation of methane to methanol, formaldehyde, or olefins. This is a difficult approach because it is complicated by significant competitive side reactions that result in low selectivities to the desired products. Moreover, the reaction must be carried out at high temperatures in the gas phase. The reaction products are more reactive than the starting material, and the competitive gas-phase reactions lead to deep oxidation. Any technological breakthrough in the direct conversion of methane could have a significant economic impact in the industrial sector.
The indirect routes for the conversion of methane involve oxidation of methane to form syngas (CO+H.sub.2) in a first stage either by steam reforming, direct partial oxidation or a combination of both, and then converted into upgraded products such as paraffins, olefins, oxygenates, waxes, and mid-distillate fuels in a second stage by the application of Fischer-Tropsch technology. The indirect route, and in particular the syngas step, is usually very energy- and capital-intensive (steam reforming is highly endothermic), operating at high pressures and temperatures. The cost of syngas production by steam reforming can be at least 60% of the integrated cost of the total plant.
Although direct partial oxidation of methane using air as a source of oxygen is a potential alternative to today's commercial steam-reforming processes, downstream processing requirements cannot tolerate nitrogen (recycling with cryogenic separations is required), and pure oxygen must be used. The most significant cost associated with partial oxidation is that of the oxygen plant. Any new process that could use air as the feed oxidant and thus avoid the problems of recycling and cryogenic separation of nitrogen from the product stream will have a dominant economical impact on the cost of a syngas plant, which will be reflected in savings of capital and separation costs.
Dense ceramic membranes represent a class of materials that offer potential solutions to the above-mentioned problems associated with natural gas conversion. Certain ceramic materials exhibit both electronic and ionic conductivities (of particular interest is oxygen ion conductivity). These materials not only transport oxygen (functioning as selective oxygen separators), but also transport electrons back from the catalytic side of the reactor to the oxygen-reduction interface. As such, no external electrodes are required, and if the driving potential of transport is sufficient, the partial oxidation reactions should be spontaneous. Such a system will operate without the need of an externally applied electrical potential. Although there are recent reports of various ceramic materials that could be used as partial oxidation ceramic membrane, little work appears to have been focused on the problems associated with the stability of the material under methane conversion reaction conditions.
Perovskites in the system La-Sr-Fe-Co-O (LSFC) have been shown to not only exhibit both oxygen ionic and electronic conductivity but also appreciable oxygen permeabilities (.apprxeq.2 orders of magnitude higher than that of stabilized zirconia) at temperatures .apprxeq.800.degree. C. These perovskites are thus a natural candidate for methane conversion, where large quantities of oxygen are required.
Ceramic materials in the family La.sub.1-x Sr.sub.x Co.sub.1-y Fe.sub.y O.sub.3-.delta. are considered promising materials for oxygen permeation at elevated temperatures. Researchers have investigated the oxygenation permeation of several materials with different values for x and y. The stability of those materials under actual gas conversion conditions however, is somewhat problematic.
A variety of formulations in the system La-Sr-Co-Fe-O have been synthesized, and reactor tubes according to these formulations have been fabricated. These tubes were tested for conversion of methane and it was found that one particular composition, SrCo.sub.0.05 FeO.sub.3-x' is more stable than any other composition in this family of materials. Reactors fabricated using this formula yielded good methane conversion results. This formulation is disclosed in U.S. patent application Ser. No. 08/212,251, filed Mar. 18, 1994, and incorporated herein by reference. Even though the material disclosed in U.S. Ser. No. 08/212,251 exhibits greater stability than other related compositions, there are certain problems associated with it. The reactor tubes fracture at regions slightly away from the hot reaction zone (the temperature of the hot reaction zone is &gt;800.degree. C., while the temperature at the failure regions is about 700.degree. C.). This may be due to slow oxygen permeation at lower temperatures through the perovskite material from air side to methane side. The slow oxygen permeation retards the refurbishing of any oxygen lost from the lattice sites in the perovskite and thus causes the material to decompose, and eventually fail by cracking.
Accordingly, it is an object of the present invention to provide a stable reactor tube composition for directly converting methane to value-added-products that is capable of using oxygen as the feed oxidant.
It is another object of the present invention is to provide a stable ceramic reactor tube for converting low hydrocarbons to high value products that exhibits greater stability when exposed to a reducing gas environment for extended time periods.
Yet another object of the present invention to provide a stable reactor tube for converting low hydrocarbons to high value products that has a compositions that exhibits both ionic and electronic conductivity as well as appreciable oxygen permeabilities.